1. Field of the Invention
The present invention relates to an adaptive optics system and more particularly to an adaptive optics system configured as an optical phase front measurement system which relies on diffraction rather than interference and includes a spatial light modulator for decoding a beam phase front and provides relatively dense phase front sampling as in holographic systems but without the need for a reference beam.
2. Description of the Prior Art
Optical signals are known to be distorted when passed through a time varying inhomogeneous medium, such as a turbulent atmosphere, ocean or biological tissue. Various optical systems are known which compensate for the distortion in a wavefront during such conditions. Such adaptive optics systems include one or more wavefront sensors for estimating the distortion of the wavefront of an optical signal. These distortion estimates are used to generate correction signals, which, in turn, are typically fed to the actuators of a deformable mirror or a spatial light modulator in order to correct for the wavefront distortion.
Various wavefront sensors are known in the art. Such wavefront sensors are known to have limitations during certain conditions. For example, both unit shear lateral shearing interferometer (LSI) wavefront sensors and Hartmann wavefront sensors are extremely well known in the art. Such sensors are disclosed, in general, for example, in xe2x80x9cPrinciples of Adaptive Optics,xe2x80x9d Second Edition, Robert K. Tyson, Academic Press, 1991, hereby incorporated by reference.
Hartmann sensors utilize a mask with a matrix of holes or an array of lenslets, for example, for dividing the wavefront into a matrix of subapertures. Each of the beams from the subapertures is focused onto one or more position sensing detectors forming an array of the spot intensity on the detectors. The location of the spots provides a direct indication of the wavefront tilt at each subaperture. Unfortunately, the number of sample points of the phase front with such Hartmann sensors is relatively sparse. As such, the applications of such Hartmann sensors are limited to relatively mild turbulent conditions.
In unit shear (LSI) wavefront sensors, a copy of the wavefront is made and shifted in the x direction by a distance equal to the spacing between the actuators in the deformable mirror. The original and shifted beams are interfered in order to find the phase difference therebetween. The interference pattern is applied to an array of detectors. The intensity of the light interference pattern provides a measure of the wavefront x-tilt. Such unit shear LSI wavefront sensors may also be implemented in the y direction to obtain the y-tilt. With such unit shear LSI wave front sensors, the sampling resolution is limited by the size of the lateral shift. Unfortunately, selecting a shift that is too small causes serious degradation in the measurement accuracy. In addition, the inevitable accumulation of measurement noise also leads to reconstruction errors.
Holographic techniques are also known for detecting the wavefront of a light beam. Such holographic techniques utilize a reference beam with a known wavefront. The reference beam is heterodyned or mixed with the unknown wavefront to obtain an interference pattern or hologram. The phase front of the unknown beam is computed from the intensity profile of the interference pattern. There are two distinct advantages of the holographic technique over the other known techniques: (1) dense sampling and (2) heterodyning gain. Unfortunately, these advantages are also significant weaknesses. More particularly, to create a hologram, the reference beam needs to be coherent with the unknown incoming wave to satisfy the Fourier condition of xcex94fxc2x7xcex94t less than  less than 1, where xcex94f is the relative frequency drift and xcex94t is the exposure time. For a continuous wave (CW) laser source, xcex94t is typically limited to the photon flight time. Commercial CW lasers with sophisticated cavity control can achieve a xcex94f down to about 300 kHz. Even then the coherence length, and thus the maximum range of operation, is still limited to only a few hundred meters. For longer range applications, a pulse laser with a xcex94t of about 10 ns may be selected. However, overlapping the return and reference pulses to within a few nanoseconds is relatively difficult in a dynamic situation. An even greater obstacle is the uncompensated Doppler shift for a moving target. If, during the time of exposure, the optical path difference varies by more than xcex/4, the hologram becomes washed out. For example, if the wavelength is 1 xcexcm and xcex94t equals 10 ns, to maintain a good contrast, the optical path difference must not vary faster than 25 m/s. Such parameters are relatively restrictive even for mobile targets. As such, laser coherence and target motion often render the holographic techniques impractical. In addition, such holographic adaptive optics systems require a reference beam. Thus, an adaptive optics system is needed which can provide relatively dense sampling of the phase fronts while eliminating the need for reference beam and allowing for relatively long range applications.
Briefly, the present invention relates to an adaptive optics system configured as an optical phase front measurement system which provides for relatively high resolution sampling as in holographic techniques but without the need for a reference beam. The optical phase front measurement system includes one or more lenses and a spatial light modulator positioned at the focal plane of the lenses and a camera which enables the phase front to be determined from intensity snapshots. The phase front measurement system allows for relatively long range applications with relatively relaxed criteria for the coherence length of the laser beam and the Doppler shift. As such, the system is suitable for a wide variety of applications including astronomy, long range imaging, imaging through a turbulent medium, space communications, distant target illumination and laser pointing stabilization.